Angular NgFor directive and trackby

Today we will learn about NgFor one of the core directive of angular. NgFor helps to build list and tables in Angular

 

Let us see the example in our flower store app. In the landing page I am going to print list of flowers that are available in the store. Later we will add images and show them in a carousel to rotate automatically.

First we will create the domain class called flower.ts and you can copy paste below code. 

export class flower{
  constructor() { }
  name:string='';
  price:number = 0;
  availableQuantity:number = 0
}

To use this domain class inside another component you must specify export keyword along with the class keyword.

Flower domain has 3 attributes to store name of the flower then price and the available quantity.

Now we can create the array from the flower type as below inside the landing.component.ts file.

myFlowerList:flower[]=[];

I will call a method inside ngOnInit to add values to the array as below.

ngOnInit is one of the life cycle hooks used in angular and it executed only one time when component is initiating.

ngOnInit() {
    this.mySellingFlowers();
  }

  mySellingFlowers(){
    let rose = new flower();
    rose.name = "Rose";
    rose.price = 100;
    rose.availableQuantity = 1000;
    this. myFlowerList.push(rose);

    let lily = new flower();
    lily.name = "Lily";
    lily.price = 80;
    lily.availableQuantity = 2000;
    this. myFlowerList.push(lily);

    let tulip = new flower();
    tulip.name = "Tulip";
    tulip.price = 100;
    tulip.availableQuantity = 2300;
    this. myFlowerList.push(tulip);

    let carnation = new flower();
    carnation.name = "Carnation";
    carnation.price = 50;
    carnation.availableQuantity = 1500;
    this. myFlowerList.push(carnation);
  }

Now we have an array with different flowers. Let’s see how we can iterate the array using ngFor directive.

Copy paste below code in you landing.component.html file.

<ul>
    <li *ngFor="let flower of myFlowerList">
        {{flower.name}}
    </li>
</ul>
 

myFlowerList is the array that we defined and initiated in the landing.component.ts file.

*ngFor is helping to loop the array element and it will hold each element in flower object and you can access it properties with the . operator.

{{}} is called interpolation and it will print the value of the variable in your browser.

Now let’s check our browser. Here it is.

Finding the index of a list element

 

Now let’s see how you can track the index of each element and use that index to generate unique id for each flower element.

<ul>
    <li *ngFor="let flower of myFlowerList;let i = index">
        <span id="flower_{{i}}">{{flower.name}}</span>
    </li>
</ul>

How to use trackBy?

Track by function is using to improve performance. If you don’t use track by function whenever your array get change it will reload the whole DOM object but with track by function it will change only that element.

 

To use trackby function you can update landing.component.html file as below

<ul>
    <li *ngFor="let flower of  myFlowerList;trackBy:trackFlowers">
        <span >{{flower.name}}</span>
    </li>
</ul>

Then define the trackFlower function in landing.component.ts file as below.

trackFlowers(index,flower){
    return flower?flower.name:undefined
}

I hope you enjoyed the post. In next post we will apply some styles and background flower images to our flower list.